CryptographicException - Bad Data when changing from SSL to no secure connection

[z f]

Hi,

in a asp-vb.net web application, i have a login page.
when i use the login page with http:// url, everything is fine,
authentication works.
when i work with https:// it also works.
when i change from https to http after the login (only login works with SSL)
i get the error
[System.Security.Cryptography.CryptographicException] - Bad Data
on the following line in the global.asax authenticateRequest

Dim authTicket As FormsAuthenticationTicket =
FormsAuthentication.Decrypt(authCookie.Value)

what can i do? please help.
TIA, z.